…ons (#811)

* Reenable FillBlockPayee for mnpayments, fix CreateNewBlock calculations

* revert blockchain.py fix #772

…tempting unlock

517c7f9 gui: Check for private keys disabled before attempting unlock (Andrew Chow)

Pull request description:

  Before trying to unlock a wallet, first check if it has private keys disabled. If so, there is no need to unlock.

  Note that such wallets are not expected to occur in typical usage. However bugs in previous versions allowed such wallets to be created, and so we need to handle them.

  Fixes dashpay#772

  For some additional context, see dashpay#631

ACKs for top commit:
  hebasto:
    ACK 517c7f9, I have reviewed the code and it looks OK.
  BrandonOdiwuor:
    ACK 517c7f9

Tree-SHA512: c92aa34344d04667b70b059d2aa0a1da999cb7239cd1413f3009781aa82379f309ff9808d7dc91d385e2c8afe2abda3564568e2091ef833b1536ebfcf80f7c3c